We had previously shown that high gene expressions (mRNA levels) of thymidylate synthase (TS; Leichman et al., J. Clin. Oncol., 15: 3223–3229, 1997)and thymidine phosphorylase (TP; Metzger et al., Clin. Cancer Res., 4: 2371–2376, 1998) in pretreatment tumor biopsies could identify tumors that would be nonresponsive to 5-fluorouracil (5-FU)-based therapy. In this study, we investigated the association between intratumoral gene expression of the pyrimidine catabolism enzyme dihydropyrimidine dehydrogenase(DPD) and the response of colorectal tumors to the same 5-FU-based protocol. DPD expressions were measured by quantitative reverse transcription-PCR in 33 pretreatment biopsies of colorectal tumors from patients who went on to receive treatment with 5-FU and leucovorin (LV). The range of DPD gene expression in those tumors that were nonresponsive to 5-FU was much broader than that of the responding tumors. None of the tumors with basal-level DPD expressions above a DPD:β-actin ratio of 2.5 × 10⁻³(14 of 33) were responders to 5-FU/LV therapy, whereas those tumors with DPD gene expressions below DPD:β-actin ratio of 2.5 × 10⁻³had a response rate of 50%. There was no correlation among DPD, TS, and TP expression values in this set of colorectal tumors, which indicated that these gene expressions are independent variables. All of the tumors that responded to 5-FU therapy (11 of 33) had expression values of all three of the genes, TS, TP, and DPD, below their respective nonresponse cutoff values,whereas, in each of the nonresponding tumors, at least one of these gene expressions was high. The patients with low expression of all three of the genes had significantly longer survival than patients with a high value of any one of the gene expressions. The results of this study show that intratumoral gene expression level of DPD is associated with tumor response to 5-FU and that the use of more than one independent determinant of response permits the identification of a high percentage of responding patients.